net: remove skb_free_datagram_locked()

Last user of skb_free_datagram_locked() went away in 2016
with commit 850cbadd

 ("udp: use it's own memory
accounting schema").

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240325134155.620531-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>